Graduate Thesis Or Dissertation
 

Effects of Tetranychus urticae Koch feeding injury on physiological processes in Mentha piperita L.

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  • Studies were conducted to examine the effects of feeding injury by the twospotted spider mite (Tetranychus urticae Koch) on physiological processes of its host plant, peppermint (Mentha piperita L.). Aspects of mite-induced host plant physiological stress that were studied included: 1) effects of injury on plant-water relations; 2) photosynthesis and leaf chlorophyll content; and 3) soluble leaf carbohydrates and starch. The effects of feeding injury are discussed in terms of underlying physiological mechanisms. In addition, new methods for sequential extraction and analysis of peppermint tissues are presented. Development of this methodology was necessary in order to satisfy the special requirements of these studies. The most detrimental effect of feeding injury was damage to leaf epidermis and cuticle and the consequent alteration of plant-water relations. Injured leaves were found to transpire more water at night than did uninjured leaves, resulting in symptoms of plant water stress the following day. These symptoms included reduced leaf water potential (psychrometrically deterimined) and stomatal closure. Levels of soluble peppermint leaf carbohydrates (mainly sucrose, raffinose, and stachyose) were higher on a per leaf basis in mite-injured leaves than in uninjured leaves. It is suggested that an osmotic adjustment mechanism, utilizing soluble carbohydrate as osmoticum, may be operating to maintain turgor in mite injury-induced water-stressed leaves. Stress-induced stomatal closure inhibited photosynthesis, presumably by restricting carbon dioxide exchange. In addition, mite feeding removed significant amounts of leaf chlorophyll, resulting in localized non-photosynthetic, necrotic patches. An injury index based on the number of feeding adult female mites, duration of feeding, and leaf area is presented. This index was used throughout the studies to estimate the relative degree of injury of infested leaves.
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